This invention relates to a method and an apparatus for etching silicon by applying an etchant containing nitric acid and hydrofluoric acid to the silicon.
For the production of a semiconductor device, it is well known to form circuits in many areas arranged in a lattice pattern on the surface of a silicon wafer, and dice the silicon wafer to separate the individual areas, thereby producing semiconductor chips. Before dicing the silicon wafer, it is common practice to grind the entire area of the back side of the silicon wafer for the purpose of improving heat dissipation of the resulting semiconductor chips and/or making them thin slices, and then etch the entire area of the back side of the silicon wafer, thereby removing strains caused by grinding. Etching is performed by applying an etchant containing nitric acid and hydrofluoric acid to the back side of the silicon wafer.
When the back side of the silicon wafer is etched with a fresh etchant, the back side of the silicon wafer can be formed into the desired mirror surface. However, the repeated use of the etchant tends to turn the etchant brown and roughen the etched back side of the silicon wafer. To maintain a high etching quality, therefore, it is necessary to use a fresh etchant without repeatedly using the etchant. This makes etching treatment considerably costly.
The object of the present invention is to provide a novel and improved etching method and apparatus which can markedly reduce the cost of etching treatment by repeatedly using an etchant at least partially, without deteriorating the quality of etching, and which are particularly suitable for, but not restricted to, etching the back side of a silicon wafer.
The inventors of the present invention conducted extensive studies, and obtained the following findings: When silicon is etched with an etchant containing nitric acid and hydrofluoric acid, an NOx (x denotes a positive integer), such as NO2 or NO, is formed. Owing to the presence of a considerable amount of such NOx, the etchant is colored brown, and the etching quality of silicon declines. When the etchant, which has been used in etching and thus contains a considerable amount of NOx, is brought into contact with a gas inert to the etchant, NOx, is removed from the etchant while accompanying the gas. Hence, the etchant can be recycled without deterioration of the etching quality.
According to the present invention, as an etching method for attaining the above-described object, there is provided an etching method comprising an etching step of applying an etchant containing nitric acid and hydrofluoric acid to silicon to etch the silicon, including:
a recovery/regeneration step of recovering the etchant used in the etching step, and bringing a gas inert to the etchant into contact with the recovered etchant to regenerate the etchant,
at least a part of the regenerated etchant after recovery and regeneration in the recovery/regeneration step being reused in the etching step.
The gas contacted with the etchant recovered in the recovery/regeneration step is preferably air. According to a preferred embodiment, in the etching step, a predetermined amount of the etchant is delivered out of the etchant accommodated in a circulation tank, and is applied to silicon, the regenerated etchant for reuse in the etching step is returned to the circulation tank, and a replenishing solution containing nitric acid and hydrofluoric acid is fed into the circulation tank for bringing at least the proportion of hydrofluoric acid in the etchant in the circulation tank to a predetermined value before the etchant is delivered from the circulation tank for a next etching step. Preferably, a predetermined proportion of the etchant regenerated in the recovery/regeneration step is discarded, and the remainder is reused in the etching step. The discarded proportion of the etchant regenerated in the recovery/regeneration step is preferably set such that an equilibrium is achieved between the amount of hexafluorosilicic acid formed in the etching step and the amount of hexafluorosilicic acid contained in the regenerated etchant discarded. The temperature of the etchant applied to the silicon in the etching step is preferably adjusted to a predetermined temperature.
According to the present invention, as an etching apparatus for attaining the above-described object, there is provided an etching apparatus for applying an etchant containing nitric acid and hydrofluoric acid to silicon to etch the silicon, comprising:
support means for supporting the silicon to be etched;
transport means for carrying the silicon to be etched onto the support means, and carrying the etched silicon out of the support means;
a circulation tank accommodating the etchant containing nitric acid and hydrofluoric acid;
etchant applicator means for applying the etchant accommodated in the circulation tank to the silicon supported by the support means;
recovery means for recovering the etchant applied to the silicon supported by the support means;
regeneration means for bringing a gas inert to the etchant into contact with the etchant recovered by the recovery means to regenerate the etchant; and
circulation means for returning at least a part of the etchant regenerated by the regeneration means to the circulation tank.
The regeneration means preferably brings air into contact with the recovered etchant. The following are preferred embodiments: The regeneration means is composed of a gas-liquid contact tower including a main section packed with many packing elements, etchant introduction means for introducing the etchant to be regenerated, from above the main section into the main section, gas introduction means for introducing a gas from below the main section into the main section, and gas discharge means for discharging the gas from above the main section. The gas-liquid contact tower includes a collection tank for collecting the etchant that has flowed downward through the main section. The etchant introduction means introduces the etchant recovered by the recovery means into the main section, and also repeatedly introduces the etchant collected into the collection tank into the main section. Replenishment means is disposed for feeding a replenishing solution containing nitric acid and hydrofluoric acid into the circulation tank for bringing at least the proportion of hydrofluoric acid in the etchant in the circulation tank to a predetermined value before the etchant applicator means delivers the etchant from the circulation tank for a next etching step. Discard means is provided for discarding a predetermined proportion of the etchant regenerated by the regeneration means. The etchant applicator means delivers a predetermined amount of the etchant from the circulation tank, and applies it to the silicon supported by the support means. Preferably, temperature adjustment means is provided for adjusting the etchant in the circulation tank to a predetermined temperature.